The invention relates generally to exhaust hoods for condensing steam turbines and more specifically to a diffuser shape within the exhaust hood.
In low-pressure steam turbines, pressure recovery for exhaust hoods can be divided into two parts: 1) pressure recovery from a diffuser inlet to end of a steam guide, and 2) pressure recovery from an end of the steam guide to a condenser. Getting pressure recovery downstream from the steam guide is very difficult as the exhaust hood contains many supporting struts after steam guide end. Consequently, any possible improvement within the steam guide should be employed.
Pressure recovery from the diffuser inlet to the end of steam guide depends on many parameters for the diffuser such as: 1) area ratio (outlet area/inlet area); 2) axial length available after last stage bucket centerline (derives turning radius); 3) last stage bucket tip leakage flows; and 4) last stage bucket shroud thickness (a larger shroud thickness creates more blockage).
The diffuser axial distance is measured as distance available from last stage bucket centerline to the end of diffuser, which is in general twice the bucket height and expressed as “2*Lbw/al”. For example, if the bucket height is 40″ then the diffuser axial length will be 80″.
For a steam turbine, reducing the axial length of the diffuser would be cost beneficial, as it directly reduces length of the rotor shaft. A shorter axial length for the diffuser, such as “1.6*Lbw/al”, requires a higher turning radius (more aggressive steam guide) to maintain required area ratio. A high turning radius will always leads to steam flow separation from the steam guide. FIG. 1 illustrates a first diffuser 10 receiving exhaust from a bucket 5 of length L with a tip shroud 6. The first diffuser 10 has a first axial length 15, a mild curvature 20 of the steam guide wall, and a first outlet area 30. A second shortened diffuser 50 is also illustrated. The second diffuser 50 includes a shorter axial length 55 with an increased outlet area 65 to maintain a same area ratio, thereby necessitating a more aggressive curvature 60 of the steam guide wall 70 that may lead to flow separation from steam guide wall.
One of the ways to reduce flow separation is using boundary layer blowoff, for example, by increasing the last stage bucket tip clearance. The jet coming from the tip clearance will reduce this flow separation, thereby leading to improved pressure recovery. But increasing the clearance is not advisable, as it will impact on last stage bucket performance.
Adding to this, a greater thickness for the shroud on last stage bucket may result in a flow blockage due to the vortex coming off the shroud. The presence of a vortex will increase losses further. FIG. 2 illustrates the effect of a tip shroud 6 on the last stage bucket 5 creating a vortex 75 within diffuser 10. The blockage by the tip shroud 5 creates a slow moving steam 70 forming the expanding slow-moving vortex 75.
Accordingly, it would be desirable to provide a means for improving pressure recovery with an aggressive steam guide.